1. Field of the Invention
The present invention relates to HARQ processing technology, and more particularly, to a HARQ operation method by considering a measurement gap. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for solving a problem caused in case that a measurement gap of interrupting uplink/downlink transmission is overlapped with a timing point of receiving HARQ (hybrid automatic repeat request) feedback information in a mobile communication system.
2. Discussion of the Related Art
First of all, 3GPP LTE (3rd generation partnership project) long term evolution: hereinafter called ‘LTE’) communication system is schematically described as a mobile communication system to which the present invention is applicable.
FIG. 1 is a schematic diagram of E-UMTS network structure as an example of a mobile communication system.
Referring to FIG. 1, E-UMTS (evolved universal mobile telecommunications system) is the system having evolved from UMTS (universal mobile telecommunications system) and its basic standardization is ongoing by 3GPP. Generally, the E-UMTS can be called LTE system.
E-UMTS network can be mainly divided into E-TRAN 101 and CN 102 (core network). The E-UTRAN (evolved-UMTS terrestrial radio access network) 101 consists of a user equipment (hereinafter abbreviated UE) 103, a base station (hereinafter named eNode B or eNB) 104, and an access gateway (hereinafter abbreviated AG) 105 located at an end point of the network to be externally connected to an external network. The AG 105 can be divided into one part responsible for user traffic processing and the other part for processing control traffic. In this case, the AG for new user traffic processing and the AG for processing control traffic can communicate with each other using a new interface.
At least one cell can exist at one eNode B. Between eNode Bs, an interface for user or control traffic transmission is usable. And, the CN 102 can consist of a node for user registrations of the AG 105 and other UE 103. Moreover, an interface for discriminating the E-UTRAN 101 and the CN 102 is available.
Layers of a radio interface protocol between a user equipment and a network can be divided into L1 (first layer), L2 (second layer) and L3 (third layer) based on three lower layers of the open system interconnection (OSI) reference model widely known in the field of communication systems. A physical layer belonging to the first layer provides an information transfer service using a physical channel. A radio resource control (hereinafter abbreviated RRC) located on the third layer plays a role in controlling radio resources between the user equipment and the network. For this, the RRC layers exchange RRC messages between the user equipment and the network. The RRC layers can be distributed to network nodes including the eNode B 104, the AG 105 and the like. Moreover, the RRC layer can be provided to the eNode B 104 or the AG 105 only.
FIG. 2 and FIG. 3 are diagrams for structures of a radio interface protocol between a user equipment and UTRAN based on the 3GPP radio access network specifications.
Referring to FIG. 2 and FIG. 3, a radio interface protocol horizontally consists of a physical layer, a data link layer and a network layer. And, the radio interface protocol vertically consists of a user plane for data information transfer and a control plane for control signal delivery (signaling). In particular, FIG. 2 shows the respective layers of the radio protocol control plane and FIG. 3 shows the respective layers of the radio protocol user plane. The radio protocol layers shown in FIG. 2 and FIG. 3 can be divided into L1 (first layer), L2 (second layer) and L3 (third layer) based on three lower layers of the open system interconnection (OSI) reference model widely known in the field of communication systems.
The respective layers of the radio protocol control plane shown in FIG. 2 and the respective layers of the radio protocol user plane shown in FIG. 3 are explained as follows.
First of all, a physical (PHY) layer of a first layer provides an upper layer with an information transfer service using a physical channel. The physical (PHY) layer is connected to a medium access control (MAC) layer on an upper layer via a transport channel. And, data is transported between the medium access control (MAC) layer and the physical (PHY) layer via the transport channel. In this case, the transport channel can be classified into a dedicated transport channel or a common transport channel according to whether a channel is shared or not. Moreover, data are transported via the physical channel between different physical layers, i.e., between a physical layer of a transmitting side and a physical layer of a receiving side.
Various layers exist in the second layer. First of all, a medium access control (hereinafter abbreviated ‘MAC’) layer plays a role in mapping various logical channels to various transport channels. And, the MAC layer also plays a role as logical channel multiplexing in mapping several logical channels to one transport channel. The MAC layer is connected to a radio link control (RLC) layer of an upper layer via a logical channel. And, the logical channel can be mainly categorized into a control channel for transferring information of a control plane and a traffic channel for transferring information of a user plane according to a type of the transferred information.
A radio link control (hereinafter abbreviated RLC) of the second layer performs segmentation and concatenation on data received from an upper layer to play a role in adjusting a size of the data to be suitable for a lower layer to transfer the data to a radio section. And, the RLC layer provides three kinds of RLC modes including a transparent mode (hereinafter abbreviated TM), an unacknowledged mode (hereinafter abbreviated UM) and an acknowledged mode (hereinafter abbreviated AM) to secure various kinds of QoS demanded by each radio bearer (hereinafter abbreviated RB). In particular, the AM RLC performs a retransmission function through automatic repeat and request (ARQ) for the reliable data transfer.
A packet data convergence protocol (hereinafter abbreviated PDCP) layer of the second layer performs a header compression function for reducing a size of an IP packet header containing relatively large and unnecessary control information to efficiently transmit such an IP packet as IPv4 and IPv6 in a radio section having a small bandwidth. This enables a header part of data to carry mandatory information only to play a role in increasing transmission efficiency of the radio section. Moreover, in the LTE system, the PDCP layer performs a security function as well. This consists of ciphering for preventing data interception conducted by a third party and integrity protection for preventing data manipulation conducted by a third party.
A radio resource control (hereinafter abbreviated RRC) layer located at a most upper part of a third layer is defined in the control plane only and is responsible for controlling a logical channel, a transport channel and physical channels in association with configuration, reconfiguration and release of radio bearers (hereinafter abbreviated RBs). In this case, the RB means a logical path provided by the first and second layers of the radio protocol for the data delivery between the user equipment and the UTRAN. Generally, configuring an RB means to stipulate characteristics of radio protocol layers and channels required for providing a specific service and also means to configure detailed parameters and operational methods thereof. The RB can be classified into a signaling RB (SRB) or a data RB DRB). The SRB is used as a path for sending an RRC message in a control plane (C-plane) and the DRB is used as a path for transferring user data in a user plane (U-plane).
As a downlink transport channel for transporting data to a user equipment from a network, there is a broadcast channel (BCH) for transmitting system information and a downlink shared channel (SCH) for transmitting a user traffic or a control message. Downlink multicast, traffic of a broadcast service or a control message can be transmitted on downlink SCH or a separate downlink MCH (multicast channel). Meanwhile, as an uplink transport channel for transmitting data to a network from a user equipment, there is a random access channel (RACH) for transmitting an initial control message or an uplink shared channel (SCH) for transmitting user traffic or a control message.
As a downlink physical channel for transmitting information transferred on a downlink transport channel to a radio section between a network and a user equipment, there is a physical broadcast channel for transferring information of BCH, a physical multicast channel (PMCH) for transmitting information of MCH, a physical downlink shared channel for transmitting information of PCH and downlink SCH or a physical downlink control (or called DL L1/L2 control channel) for transmitting control information provided by first and second layers.
As an uplink physical channel for transmitting information forwarded on an uplink transport channel to a radio section between a network and a user equipment, there is a physical uplink shared channel (PUSCH) for transmitting information of uplink SCH, a physical random access channel (PRACH) for transmitting RACH information or a physical uplink control channel (PUCCH) for transmitting such control information, which is provided by first and second layers, as HARQ ACK, HARQ NACK, scheduling request (SR), channel quality indicator (CQI) report and the like.
Based on the above description, HARQ processing performed in the LTE system is schematically explained as follows.
FIG. 4 is a diagram for HARQ operation performed in the LTE system.
Referring to FIG. 4, a terminal (UE) is set to a transmitting side and a base station (eNode B or eNB) is set to a receiving side. Assume an uplink situation that HARQ feedback information is received from the base station. This is identically applicable to a downlink situation as well.
First of all, in order to enable a terminal to transmit data, a base station is able to transmit uplink scheduling information, i.e., uplink (UL) grant via PDCCH (physical downlink control channel) [S401]. In this case, the UL grant can include a terminal identifier (e.g., C-RNTI, semi-persistent scheduling C-RNTI), a location of an assigned radio resource (resource block assignment), a transmission parameter such as a modulation/coding rate, a redundancy version and the like, a new data indicator (NDI), etc.
The terminal is able to check UL grant information sent to itself by monitoring PDCCH each TTI (transmission time interval). In case of discovering the UL grant information sent to itself, the terminal is able to transmit data (‘data 1 in FIG. 4) on PUSCH (physical uplink shared channel) according to the received UL grant information [S402]. In this case, the transmitted data can be transmitted by MAC (medium access control) PDU (protocol data unit).
As mentioned in the above description, after the terminal has performed the uplink transmission on PUSCH, it stands by for HARQ feedback information reception on PHICH (physical hybrid-ARQ indicator channel) from the base station. If HARQ NACK for the data 1 is transmitted from the base station [S403], the terminal retransmits the data 1 in a retransmission TTI of the data 1 [S404]. On the contrary, if HARQ ACK is received from the base station [not shown in the drawing], the terminal stops the HARQ retransmission of the data 1.
Each time the terminal performs one data transmission by HARQ scheme, the terminal takes a count of the number of transmissions (CURRENT_TX_NB). If the transmission number reaches a maximum transmission number (CURRENT_TX_NB) the terminal discards MAC PDU stored in HARQ buffer.
If HARQ ACK for the data 1 retransmitted in the step S404 from the terminal is received [S405] and if a UL grant is received on PDCCH [S406], the terminal is aware of whether data to be transmitted this time is an initially-transmitted MAC PDU and whether to retransmit a previous MAC PDU using a new data indicator (NDI) field received on PDCCH. In this case, the NDI field is a 1-bit field. The NDI field is toggled as 0→1→0→1→ . . . each time a new MAC PDU is transmitted. For the retransmission, the NDI field is set to a value equal to that of the previous transmission. In particular, the terminal is ware of whether to retransmit the MAC PDU by comparing the NDI filed is set to a previously-transmitted value.
In case of FIG. 4, as a value of ‘NDI=0’ in the step S401 is toggled into ‘NDI=1’ in the step S406, the terminal recognizes that the corresponding transmission is a new transmission. The terminal is then able to transmit data 2 on PUSCH [S407].
Meanwhile, in the LTE system, a base station is able to set up a measurement operation for a terminal which needs inter-measurement for mobility support. Thus, in a measurement gap for which the terminal performs the inter-measurement, a communication between the base station and the terminal is interrupted in general. In this case, the ‘inter-measurement’ includes intra-frequency measurement, an inter-frequency measurement, inter-RAT mobility measurement, etc. The ‘inter-measurement’ may be called as a ‘measurement gap operation’ if it does not cause any confusion.
The interval of the measurement gap may be determined according to a setup of the base station. As the measurement gap operation is performed each the determined interval, the terminal stops transmission to the base station in uplink for 6˜7 ms and stops reception in downlink for 6 ms.
However, if the measurement gap coincides with the HARQ feedback reception timing, it is impossible for the terminal to receive the HARQ feedback from the base station.